How can I be sure the berries I purchase are of superior quality?

Contrary to popular belief, strawberries to do not continue to ripen after picking. Ripe strawberries are bright red, plump, and carry a sweet scent. They are also shiny. Unripened strawberries look white and bland inside. Check the bottom of the package for moisture or strawberry juice--a sign of damaged fruit. Damaged strawberries only last for hours and can affect the wholesomeness of other berries in the package.
Blueberries continue to ripen after picking. Select firm, dry blueberries that have a subtle, whitish bloom rather than a uniform blue-black color. The bloom is a sign of freshness, not mold. Gently shake the carton to make sure the berries move freely. If they don't, they may be soft and damaged. Avoid blueberries that are soft, watery, or shriveled in appearance.
Raspberries do not continue to ripen after picking. Look for raspberries that are plump, firm, dry, and deeply colored. Avoid berries that exhibit moisture or loss of color.  
 
Source(s):
Boise State University "Have You Ever Wondered How to Tell If Fruit is Ripe?"
Missourifamilies.org "Selecting Blueberries"
Everything2.com "How to Tell If Your Fruit is Ripe"
Sunnyridge.com "Strawberries"
Wholefoodsmarket.com "Guide to Fruits" - Blueberries
Wholefoodsmarket.com "Guide to Fruits" - Raspberries
Doleeurope.com "Raspberries"